NEW GALLIUM END-MEMBER IN EPIDOTE GROUP 1
2
V41B-2501
1
Anna Soboleva , Dmitry Varlamov , Tatyana Mayorova 1
Institute of Geology, Komi Science Centre, Ural Division, Russian Academy of Sciences, Syktyvkar, Russia, 167982,
[email protected],
[email protected] 2 Institute of Experimental Mineralogy, Russian Academy of Sciences, Chernogolovka, Russia, 142432;
[email protected] Financial support by Russian Fund of Basic Researches, grant №11-05-01087-a Unique complex-zoned ultrahigh-Ga (Ga2O3 up to 20 wt. %) mineral of epidote group is discovered in Tykatlova gold-sulfure ore occurrence in the Subpolar Urals, Russia. It is the first find of the Ga silicate mineral in the world.Minerals with species-defining gallium are extremely rare. High content of gallium in other natural phases also very exotic. Due to affinity of chemical properties with the main rock-forming elements (Al, Fe, Zn) and wide opportunities of isomorphism with them, gallium does not form the significant concentrations, despite considerable Clarke content (17-19 ppm) which is more than at the majority of rare elements. Only Rb and Re have less number of minerals (among stable non-gas elements). Only 5 Ga minerals are presented in the IMA official list. Generally, only two unique deposites in Africa contain small amount of Ga minerals – Tsumeb in Namibia and Kipushi in DR Congo. Location ang geological scheme of Tykatlova deposit
Vorkuta
KOMI REPUBLIC
1
2
3
Ukhta
Tykatlova ore occurrence is situated in Early Ordovician acid volcanic rocks (rhyolites, rhyodacites) metamorphosed in greenshists facies and is located in zones of schistosity and secondary silification with superimposed sulfide mineralization. Ga-phases have been found out in the sphalerite-pyrite veins with galena cement and subordinated quantity of chalcopyrite. Mineral composition: Ore minerals – sphalerite+pyrite association with galena cement and subordinated amount of chalcopyrite. Minor ore minerals – Ag-bearing bornite, tetrahedrite-tennantite, different silver and copper sulfides and sulfosalts, small amount of gold, secondary ore minerals – anglesite, cerussite, lead and zinc hydroxides Veiny minerals – quartz (strongly prevails, sometimes there is only it), calcite, Zn-Mn carbonates, anhydrite or hypsum, epidote (+epidote-(Pb)) Massive ores
Vein and banded ores
4
5
Very complicate zoned Ga-phases were found out in sphalerite-pyritegalena assemblage with chalcopyrite. Ga-minerals are usually inclusions (common in sphalerite (1), sometimes in pyrite (2) or galena(3), they are rarely located between grains of sulfides (4) or non-ore ninerals (5). Chemical composition of different zones is present in Table (accordingly letters on photos a-d)
Raman spectroscopy 25000
12000
10000 20000
Typical chemical compositions (simplified) SiO2 34.76
Zone «?» “Epidote-(Ga)"
Al2O3 19.02
CaO 20.50
FeO* 2.72
MgO 0.86
MnO 0.22
Ga2O 3 19.02
8000
measured 15000
?REE 0.16
34.84
18.91
20.98
3.70
1.20
0.38
18.06
0.11
37.12
20.51
22.46
4.05
0.88
0.17
13.79
0.04
6000
10000 4000
5000 2000
36.29
Zone «B» High-gallium allanite-(Ce) Zone «C» Allanite-(Ce) with low content of Ga or without Ga Zone «D» Epidote-allanite rims with low REE and Ga
31.67
20.68 15.53
21.78 13.10
4.24 4.66
1.57 2.51
0.33 2.48
11.59 12.64
0.10 13.69
epidote 0 300
400
500
600
700
800
900
34.80
18.81
17.01
7.05
0.95
1.10
10.20
9.60
32.02
15.73
13.15
6.48
2.15
1.85
8.07
17.02
33.91
18.64
15.36
10.92
2.10
0.78
4.48
9.87
33.78
18.46
15.70
12.39
1.86
0.69
2.14
11.81
35.46
22.43
17.05
9.50
2.58
0.81
1.08
7.53
36.79
17.43
11.61
10.66
2.12
0.69
0.08
17.19
35.39
23.39
17.77
8.77
2.32
0.46
0.00
8.59
Epidote
38.84
25.11
22.52
11.72
1.52
0.37
0.85
0.00
Ca2Al 2Fe3+[Si2O7][SiO4]O(OH)
37.51
23.50
21.58
11.41
1.92
0.44
0.54
2.31
37.68
23.72
22.25
11.47
1.60
0.36
0.12
0.60
37.14
23.67
21.64
10.41
1.88
0.37
0.00
1.46
1100
Reference spectra are taken from the RRUFF project (http://rruff.info)
Proposed crystallochemistry scheme
R(Fe3-Ga3)=-0.92 Fe3+ « Ga3+ (M3 position)
Calculated formulae of end-member - “Gallium Epidote” 3+ Ca1.85Mg0.11Mn0.02)1.98(Al1.89Ga1.03Fe 0.16)3.08(Si2.93Al0.07)3O12(OH) 3+ (Ca1.88Mg0.15Mn0.03)2.06(Al1.77Ga0.97Fe 0.26)3.00(Si2.91Al0.09)3O12(OH)
1000
0 1200
“Galloepidote” Ca2 Al2Ga[Si2O7][SiO4]O(OH)
Fe3+ + Ca2+ « Fe2+ + REE3+
Ga3+ + Ca2+ « Allanite-(Ce) Fe2+ + REE3+ CaCeAl2Fe2+[Si2O7][SiO4 ]O(OH)
